The burden of the demands for improved performance of computers and other electronic devices falls on the lithographic processes used to fabricate integrated circuit chips. Lithography involves irradiating a mask and focusing the pattern of this mask through an optical microlithography system onto a wafer coated with a photoresist. The pattern on the mask is thereby transferred onto the wafer. Decreasing the line-widths of the features on a given wafer enables the writing on a wafer of more elements per unit area and consequently brings about advances in performance. The enhanced resolution required to achieve finer line-widths is enabled by decreasing the wavelength of the illumination source. As a result of the desire to achieve finer line widths, the energies used in lithographic patterning are moving deeper into the UV region. Consequently, optical components capable of reliable performance at these increasingly shorter optical microlithography wavelengths are required.
Few materials are known that have a high transmittance at wavelengths below 200 nm, for example, at 193 nm and 157 nm, and also do not deteriorate under exposure to intense laser radiation. Fluoride crystals such as those of magnesium fluoride, calcium fluoride and barium fluoride are potential materials with high transmittance at wavelengths <200 nm. Calcium fluoride crystals are particularly preferred for making optical elements for use at below 200 nm wavelengths. However, the commercial use and adoption of below 200 nm wavelengths has been hindered by the transmission nature of such deep ultraviolet wavelengths through optical materials and by the lack of economically manufacturable, high quality blanks of optically transmissive materials suitable for below 200 nm microlithography optical elements. Many factors go into making metal fluoride crystals and elements suitable for use in below 200 nm lithography. One of these is the purity of the metal fluoride starting material or feedstock used to make a metal fluoride single crystal and subsequently an optical element.
The transmission of a metal fluoride crystal is greatly dependent on the purity of the starting material. Commercially available raw material powders are sufficiently pure in terms of cationic impurities, but these materials are not sufficiently pure in terms of the anionic impurities they contain. In order to remove these anionic impurities, mainly oxygen containing species, a purification step is carried out using a gaseous or a solid oxygen scavenger, or both, such as known in the art. Examples of such scavengers are solid lead fluoride (PbF2), zinc fluoride (ZnF2) or gaseous carbon tetrafluoride (CF4). There are numerous patents and technical articles whose authors discuss the detrimental effects of water and oxygen, and the necessity for adding an oxygen scavenger to remove these contaminants. For examples, see:    J. M. Ko et al., “Czochralski growth of UV grade CaF2 single crystals using ZnF2 additive as scavenger”, J. Crystal Growth 222 (2001) 243–248 [with references to the use of PbF2 and CF4 as scavengers];    R. C. Pastor, “Crystal growth of metal fluorides for CO2 laser operation. I. The necessity of the RAP approach”, J. Crystal Growth 200 (1999) 510–514, and “Crystal growth of metal fluorides for CO2 laser operation. II. Optimization of the reactive atmosphere process (RAP) choice.”, J. Crystal Growth 203 (1999) 421–424;    J. T. Mouchovski et al., “Growth of ultra-violet grade CaF2 crystals and their application for excimer laser optics.”, J. Crystal Growth 162 (1996) 79–82;    K. A. Becraft et al., “In Situ Vibration Spectroscopic Studies of the CaF2/H2O Interface”. Langmuir 2001, Vol. 17, pages 7721–7724;    Yutong WU et al., “X-ray Photoelectron spectroscopy Study of Water Absorption in BaF2 (111) Surfaces”, Langmuir 1994, Vol. 10, pages 1482–1487;    V. M. Bermudez, “Study of Absorption on Radiation-Damaged CaF2 (111) surfaces”, Applied Surface Science, 2000, Vol. 161, pages 227–239;    N. H. de Leeuw et al., “Density functional theory calculations of adsorption of water at Calcium Oxide and calcium fluoride surfaces”, Surface Science (May 1, 2000), Vol. 452 (1–3), pages 9–19;    M. Reichling et al., “Degradation of the CaF2 (111) surface by air exposure”. Surface Science (Sep. 20, 1999), Volume 439 (1–3) Pages 181–191; and    M. Reichling et al., “Electron- and photon-stimulated metallization and oxidation of the CaF2 (111) surface”, Surface Science (May 15, 1998), Vol. 404 (1–3), pages 145–149.
See also U.S. Pat. Nos. 4,379,733 and 6,238,479 B1, 6,270,570, 6,364,946 B2; European Patent Application Publication No. EP 1 234 898 A1; and PCT Patent publication WO 02/063076 A1.
In a typical process for removing oxygen-containing anionic impurities from a metal fluoride raw material, a metal fluoride powder is melted in the presence of an oxygen scavenger. The scavenger reacts with the oxygen containing species and converts it to a fluoride material. The resulting melt or liquid is then quickly cooled down in order to obtain a purified, high density metal fluoride material that will be used in crystal growth furnaces to fabricate the desired metal fluoride crystal. This specific purification-melting process of the raw material powder is generally called a “premelt” step or “cullet” production.
For below 200 nm lithographic optical material, the quality of the premelt material may be quantified by determining the transmission characteristics of the premelt material in the wavelength range of 120–220 nm. In the absence of specific absorptions in VUV range at 130 nm, 150 nm and 193 nm, the premelt material is characterized as being an oxygen-free or substantially oxygen free material and hence suitable for making metal fluoride single crystals. Once the premelt's suitability for use in growing crystals has been determined, the premelt material is broken into pieces or ground into fine particles and loaded into crucibles that are placed in a crystal growth furnace. However, if the oxygen-free premelt material is loaded into the crucible without the addition of an oxygen scavenger, or if the crystal growth is carried out in the absence of a gaseous oxygen scavenger, the crystal produced in the growth furnace will exhibit the characteristic oxygen absorption bands in the 120–220 nm range. The presence of these bands indicates that the oxygen-free premelt material has been re-contaminated by the formation of oxygen containing substances during handling and/or during the crystal growth process. The sources of contamination can be oxygen containing substances (for example, H2O, O2, CO2 and other oxygen containing species) adsorbed on the surface of the premelt particles while they are being prepared for the crystal growth process or they can come from oxygen containing substances released from the materials used to make either the furnace or the components used in the crystal growth process. For example, the oxygen containing substances can be released from the crucibles, heating elements, and insulating parts of the furnace among others. In any event, when the furnace is heated in the absence of an oxygen scavenger, metal fluoride premelt material fluoride reacts with these oxygen containing substances and this in turn gives rise to a low transmittance crystal. For example, if water vapor is released from the insulation, the vapor can react with the metal fluoride to form a hydroxy metal fluoride. For example, CaF2+H2O→Ca(OH)F+HF↑ or CaO+2HF↑.
While the use of an oxygen scavenger during the crystal growth process can alleviate the oxygen contamination problem, such use can give rise to additional problems. For example, one must estimate the amount of contaminants present in the material and use sufficient scavenger to remove all of it. This is frequently difficult to determine. As a result, excess oxygen scavenger is used in the crystal growing process. While the use of excess scavenger does not present problems when the scavenger is gaseous, when a solid scavenger such as PbF2 is used, the excess of scavenger could result in lead contamination of the grown crystal. If insufficient scavenger is used, then residual oxygen containing substances may be present in the grown crystal and these substances will absorb radiation in the 120–220 nm regions, and consequently interfere with the transmission properties of the crystal and optical element made from it. Transmission of 96%/cm or greater at 157 nm and 99%/cm or greater at 193 nm of incident radiation is required for elements used in below 200 nm lithography.
Therefore, in view of the foregoing problems it is desirous to be able to identify other process of removing oxygen containing species from the metal fluoride materials used in the preparation on metal fluoride single crystals. Accordingly, the present invention is directed to the use of a halogen containing plasma to remove oxygen containing substances present in either a metal fluoride feedstock or arising from the surfaces or from the materials present in the interior of a crystal growth furnace.